An image sensor device is a semiconductor device with the capacity to convert an optical image into an electrical signal. Image sensor devices are used in a variety of imaging applications including medical products, navigational equipment, and consumer products such as digital cameras and cellular phones.
Many systems include image sensor devices to sense and capture optical images that can be electronically converted to a digital representation of the image. Image sensor devices include an array of photo-sensitive devices such as photodiodes or photo-transistors fabricated on, for example, a complementary metal oxide semiconductor (CMOS) substrate. Each photo-sensitive device is sensitive to light in such a way that it can create an electrical charge that is proportional to the intensity of light striking the photo-sensitive device. The overall image captured by an image sensor device includes many pixels arranged in an array such that each pixel detects the light intensity at the location of that pixel.
Image sensor devices fabricated according to a conventional CMOS process are known as CMOS imagers and may be configured to include active pixel sensors (APS). An active pixel sensor (APS) includes an integrated circuit containing an array of pixels, each containing a photo detector (e.g. photodiode or other similar device) as well as other transistors for resetting and gating the stored charge on the photo detectors. In a conventional CMOS imager, each pixel cell in an array of pixels operates to convert light intensity to electrical charge, accumulate the electrical charge in proportion to the light intensity, and transfer the accumulated charge to an amplifier. In many CMOS imagers, a pixel may be reset to a specific reference voltage level prior to, or after, acquiring the image.
Conventional image sensor devices, in various configurations, may comprise a pixel array formed in a p-region over an n-type semiconductor substrate tied to a positive voltage or, alternatively, may comprise a p-region over an n-epi (epitaxial) or n-type implanted layer that is tied to a positive voltage and formed over a p-type substrate. One purpose of the aforementioned configurations is to provide for a barrier region to reduce dark current and cross-talk between adjacent pixels in a pixel array. An adverse side effect of utilizing an n-type substrate, or a p-type substrate with an n-epi or n-type implanted layer is that the pixel array lacks a substrate to act as a ground conductor and, therefore, the only ground conductor within the pixel array is the surface p-type region with a ground strap located on the outer edge of the pixel array. As a result, these conventional designs experience a large resistance drop on the ground plane from the edge of a pixel array to the center of the pixel array. Although these conventional designs have been successful in their intended function of decreasing dark current and cross-talk between adjacent pixels in the pixel array, they have increased the ground resistance of the pixel array.
FIGS. 1(a) and 1(b) illustrate an output response across a cross-section of a conventional image pixel array utilizing an n-substrate, or alternately, an n-epi or implanted n-type layer formed over a p-type substrate. Due to the increased ground resistance caused by the lack of a sufficient ground connection across the pixel array, pixels at the center of the array may display a lower response than pixels located at the edges of the array. Therefore, the output response across the pixel array may experience a dip 104 (dark spot in the image) in the center of the array as shown by FIG. 1(a). Alternatively, pixels at the center of the array may display a higher response than pixels located near the edges of the array, and therefore, the output response across the pixel array may experience a peak 102 (bright spot in the image) in the center of the array as shown by FIG. 1(b).
There is a need for methods, apparatuses, and systems to improve the quality of an image sensor device. Specifically, there is a need for improving the layout of a pixel array by maintaining electrical and optical isolation of adjacent pixels of a pixel array while providing a sufficient ground connection across the pixel array, and decreasing the ground resistance of the pixel array.